In a Redundant Arrays of Inexpensive Disks (RAID) system, a controller module (CM) as a control device and a redundant CM in other system notify each other of various states in each of the CMs using inter-CM communication. For example, Inter-Integrated Circuit (I2C) bus is used for the inter-CM communication.
FIG. 6 is a diagram of the inter-CM communication using the I2C bus. As illustrated in FIG. 6, the CM includes a Field Programmable Gate Array (FPGA). The FPGA includes a master port and a slave port. The CM is made redundant, and the inter-CM communication is performed using the I2C bus between the master port in one system and the slave port in the other system.
The I2C bus is used to transmit and receive data using a clock output from the master port in the one system. Therefore, if there occurs any device fault or stuck fault in which a signal level of the clock is stuck to a specific level due to abnormality in a signal line, subsequent communication including data transmission/reception becomes impossible. In this way, the impossibility of communication is called “deadlock”.
FIG. 7 is a diagram of an example of how the deadlock occurs. As illustrated in FIG. 7, when a stuck fault occurs in a clock signal, the signal level of the clock signal does not change, and this causes subsequent data transmission/reception to become impossible.
When Serial Data Access (SDA) indicating a signal line for data is in an abnormal state in which the SDA is stuck to a fixed value of a low level, then the state in which input/output data is kept to the fixed value and does not change therefrom is detected. A technology is known in which, when the state where input/output data is kept to a fixed value and does not change therefrom continues for a given period of time, then it is determined that the I2C bus is in an abnormal state (see Patent Literature 2).
Patent Literature 1: Japanese Laid-open Patent Publication No. 2008-197752
Patent Literature 2: Japanese Laid-open Patent Publication No. 2002-063080
Patent Literature 3: Japanese National Publication of International Patent Application No. 2009-535677
Patent Literature 4: Japanese National Publication of International Patent Application No. 11-502643
However, the inter-CM communication using the I2C bus has a problem in which the abnormal state of the I2C bus is not detected. As an example, the abnormal state that the I2C bus is stuck to a specific level is not detected in the inter-CM communication.
The problem arises not only in the RAID system but also in components that communicate with each other using signals, such as Central Processing Unit (CPU) and a memory.